These systems must increasingly comply with spurious emission standards. In fact, with the sharing of the radiation spectrum between the different civil and military domains, it has become crucial not to interfere with the adjacent frequency bands.
Compliance with these standards is a problem, particularly for solid-state transmission systems comprising transmitters based on microwave frequency transistors or microwave frequency transistor modules. In fact, these transistors are current-switched or voltage-switched to create the microwave frequency transmission pulses, and the switched edges generate spurious emissions.
A known solution for pulse radars is to flatten the temporal edges in order to obtain a pulse with soft edges on the rising edge and falling edge in order to substantially reduce transmission spectrum congestion. The theory shows that square pulses cause sinc harmonics. With the former travelling-wave tube radars, the rising and falling edges of the pulse were modelled in a pseudo-Gaussian manner by using, for example, variable capacitances in the modulator.
When microwave frequency transistors are used, the energizing of these elements often causes increased amplitude oscillation which is highly detrimental and which is not controllable.
One of the known solutions is to modulate the power supply of the microwave frequency transistor during transmission. One disadvantage of this solution is that it creates strong currents for very short periods of time. Highly sophisticated power supply controls and a substantial amount of electronics are therefore needed. Microwave frequency power transistors often operate in class C. The modulation of their power supply voltage, if it is possible, is not guaranteed by the manufacturer. Furthermore, due to the decoupling capacitances of the transistors, the modulation of the power supplies on the falling edge is difficult to control.
The other known solutions use the linear amplification zone for the amplitude modulation of the pulse. This modulation is performed by modulating the level of the driving signal of the microwave frequency transistors within the limit of their linear operation.
These solutions are limited to the linear zone of the transistors and do not allow the waveforms to be optimized. They impose a linearity on the entire chain from the waveform generation to the amplification. Furthermore, efficiency is lower in this case and these modulations often cause a heat dissipation which takes place in the components or loads.